In many circuit arrangements, a logic signal must be transmitted between two circuits that must otherwise be electrically isolated from one another. For example, the transmitting circuit could utilize high internal voltages that would present a hazard to the receiving circuit or individuals in contact with that circuit. In the more general case, the isolating circuit must provide both voltage and noise isolation across an insulating barrier. Such isolation circuits are often referred to as “galvanic isolators”. One class of galvanic isolators is based on transforming the logic signal to a light signal that is then transmitted to an optical receiver in the receiving circuit that converts the optical signal back to an electrical signal.
Galvanic isolators based on one or more electrical transducers have also been developed. One example is an opto-coupler. In these galvanic isolators, the transmitter drives the LED to generate a light signal that is received by an optical receiver that is located on the other side of the isolation barrier. Typically, the transmitter and the LED are constructed together while the receiver and optical receiver are constructed on a separate chip.
In a number of applications of interest, multiple logic signals must be transmitted across the isolation barrier. In principle, a separate galvanic isolator could be used for each signal. However, the cost of the galvanic isolators is a significant issue in many of these applications. In addition, the space required for the multiple isolators on the printed circuit boards is also a problem in some applications.
In principle, the individual data streams could be combined to provide a single data stream in which the data is time-domain multiplexed and sent over a single isolation channel. In such schemes, the various data streams are received on separate lines of an input circuit that samples the individual lines at a predetermined rate. The sampled values are combined into a digital “frame” having one slot for each data stream. The frame is then sent as a data packet over the isolation channel. At the receiving end, the frame is unpacked and the sampled values are placed on the individual data output lines.
For this strategy to function, the input circuit needs to recognize the beginning of a data packet. Hence, some form of unique sequence of signals must be provided to mark the beginning of a data packet. The costs associated with providing the start signal across the isolation barrier are prohibitive in some applications. In addition, the receiver must have some form of clock that is sufficiently synchronized with the clock in the transmitter to allow the receiver to determine where the data bits in the frame begin and end. The cost of providing high precision clocks in both the transmitter and receiver is also a problem in some applications.